In recent years, nonaqueous-electrolytic-solution secondary batteries having high voltage and high energy density have become widely used as power sources. In order to make effective use of such high-density electricity storage devices, attempts are being made to make more efficient use of energy by employing capacitors capable of high-speed charging/discharging as power buffers. Various nonaqueous-electrolytic-solution hybrid electricity storage devices including lithium ions in the nonaqueous electrolytic solution have been proposed as capacitors having a large electricity storage amount.
As for nonaqueous-electrolytic-solution secondary batteries, various additives for nonaqueous electrolytic solutions have been proposed in order to improve the stability and electric characteristics of the nonaqueous-electrolytic-solution secondary batteries. Examples of additives that have been proposed include 1,3-propanesultone (see, for example, Patent Literature 1), vinylethylene carbonate (see, for example, Patent Literature 2), vinylene carbonate (see, for example, Patent Literature 3), 1,3-propanesultone and butanesultone (see, for example, Patent Literature 4), vinylene carbonate (see, for example, Patent Literature 5), and vinylethylene carbonate (see, for example, Patent Literature 6). Among the above, vinylene carbonate has been widely used because of its excellent effect. These additives form a stable film called a solid electrolyte interface (SEI; solid electrolyte film) on the surface of the anode. It is thought that covering the anode surface with this film suppresses the reduction/decomposition of the nonaqueous electrolytic solution.
Unfortunately, little has been proposed regarding electrolytic-solution additives having an excellent effect on hybrid electricity storage devices that employ activated carbon as the cathode and carbon materials used in lithium-ion secondary batteries as the anode.